1. Field of the Invention
This invention relates to a sacral fixation device. More particularly, this invention relates to a self-tapping cancellous screw for insertion into the sacrum.
2. Description of the Prior Art
As far as the present inventor is aware, the only devices designed for posterior sacral fixation are sacro-alar hooks and Knodt rods. The sacro-alar hooks sold by Zimmer have a construction similar to that of a conventional spinal hook in that these hooks comprise a body, a hook shoe connected by a connecting portion. However, in sacro-alar hooks, the connecting portion has a much greater length so that it fits over the superior aspect of the sacral ala. Since sacro-alar hooks can only push down or caudally against the sacrum, they can be used only with distraction rods. As such, conventional sacro-alar hooks suffer from the following disadvantages. First, considerable additional dissection is needed to insert the hooks and attach distraction rods since they are both lateral and anterior to the area usually dissected in performing a spinal fusion. This causes increased bleeding and takes additional time. Furthermore, misplacement of the hooks may result, leading to irritation of nerves if the hooks are inadvertently placed in the sacral foramina or loss of fixation if they are not placed far enough anteriorly on the ala. Second, since the sacro-alar hooks do not fix directly into the sacral bone, when loaded in distraction, the hooks tend to rotate posteroinferiorly, thereby losing both distraction and rigidity of fixation. Third, since the hooks are placed far lateral to the usual position for spinal rods, they cannot be used with spinal rod sleeves and cannot be wired to the lumbar lamina in order to prevent loss of lumbar lordosis. Hence, such hooks are associated with a high incidence of symptomatic iatrogenic kyphosis, about 40% of all cases.
The only other alternative for posterior sacral fixation is the Knodt rod which is a threaded turnbuckle with a small sharp hook on each end. This device is designed for distracting between the upper edge of the sacral spinal canal and the L-4 lamina. Customarily, two rods are used. The lower hooks slide under the thin bone which covers the spinal canal at the top edge of the sacrum. Its primary purpose is to attain some internal fixation with the hope of decompressing nerve roots and facilitating bony fusion. Similar to the alar hooks, the Knodt rod can be used only with distraction rods.
Knodt rods suffer from the following disadvantages. First, they force the lumbosacral spine into flexion creating iatrogenic kyphosis and loss of normal lumbar lordosis. Second, available data suggests that as a method for facilitating fusion, they offer no advantage over use of no internal fixation This is probably because the undifferentiated mesenchymal cells which must achieve the spine fusion are encouraged to form bone under compression and fibrous tissue under distraction. Also, they do not provide rigid fixation. Knodt rods act as uniplanar jacks and thus achieve no side-to-side or rotational stability. Third, the small sharp Knodt rod hooks frequently cut out of the thick sacral bone and/or lose position due to their limited degree of fixation onto the sacrum and curved shoe shape. Fourth, the use of hooks into the sacral canal causes nerve root impingement or injury to the dura (lining of the spinal cord and nerves) in occasional cases. This is because the sacral canal is very narrow in the anterior posterior plane so that the hooks press upon the dura. This may irritate nerves and cause pain or muscle dysfunction. The sharp hooks can also erode through the dura.
In addition to the above, several spinal screws have been designed to fix either cables or rods to the vertebral bodies on the anterior aspect of the spine. In general, these screws comprise a housing attached to a threaded shank. However, when such screws are inserted in the posterior aspect of the sacrum, they would not provide a satisfactory method of fixation for spinal rods for two reasons. They contain no articulation for accommodating the variable angle between the sacrum and lumbar spine. Also, the hole in the housing is too anterolateral (or close to the sacrum and facets) to line up with a spinal rod. Moreover, this position would preclude the use of spinal rod sleeves. All existing spinal screws are designed for anterior spinal fixation which involves a completely different type of surgery and cannot be used as an alternative to the present sacral fixation screw. Fixation into the anterior aspect of the lumbar spine involves operating on the opposite or front side of the patient and generally has very different purposes from those operations encompassed in the present invention.
The only other devices which may be used to stabilize the lumbosacral junction are rods which fit into the iliae. The iliae are pelvic bones which articulate with the sacrum at the sacroiliac joint. The two devices in this category include Harrington .TM. sacral rods and Luque rods. However, the Harrington sacral rods are not fixed into the sacrum but rather into the iliae. As a result, they have the following four disadvantages. First, the rods involve extensive lateral dissection beyond the sacroiliac joints in order to bolt into the two iliae and place the distal hooks onto the sacral bar. Second, the rods can be used with distraction rods only and have no provision for compression rod fixation. Third, the very posterior location of the iliac rod forces the spine into kyphosis to a greater extent than any other fixation device which eliminates normal lumbar lordosis. Fourth, the iliac rods fix the lumbar spine to the pelvic iliac bones thus fixing the sacroiliac joints in addition to the lumbosacral junction. Animal studies have shown that internal fixation across normal joints can lead to arthritic degeneration. Following a successful lumbosacral fusion, there is probably more than normal stress imparted to the sacroiliac joints. It is most unfortunate therefore, that this system must internally fix across the sacroiliac joints in order to achieve some lumbosacral junction fixation.
As to Luque rods, these are sometimes inserted into the iliac bone in an effort to achieve some fixation of the lumbosacral junction. Since the rods are contoured and wired to the lumbar lamina this method does not necessarily cause a loss in normal lumbar lordosis. Its liabilities include: (1) inadvertent sacroiliac fixation; (2) inability to either achieve compression or distraction across the spinal segments under treatment; and (3) the need to pass wires under the lamina and next to the dura so as to affix the rods to the spine. This method achieves no direct sacral fixation.
From the above discussion, it is clear that the presently available devices for sacral fixation suffer from numerous disadvantages. The present invention has the objective of overcoming such disadvantages and providing a sacral fixation device which is useful in all types of posterior spinal surgery.